The present invention relates generally to a method of assembling an apparatus and, more particularly, to a method of assembling fuel injectors.
The advent of the unit fuel injector addressed a basic problem encountered by the prior art; namely, runs of separate high-pressure fuel lines from a fuel pressurization means to an injection nozzle. The unit injector solved this problem by incorporating the high pressure fuel pump and the injection nozzle into a single unit. The unit injector must be capable of carrying highly-pressurized fuel. Moreover, the unit injector must also be capable of operation at very high cycle rates. Therefore, to control performance parameters such as the timing of the fuel injection and the delivery characteristics of the fuel injection with the needed level of precision, the parts of the unit fuel injector had to be manufactured and assembled with extremely precise tolerances.
Early attempts at controlling the performance variability associated with dimensional tolerance variation involved a post-assembly adjustment of preselected mechanical components in the injector. However, this solution was not entirely satisfactory due to the variability of the adjustment itself. A later approach to this manufacturing problem involved what is now known as the select fit process. This process recognized that the dimensional tolerances associated with the components involved in the manufacture of a unit fuel injector are so exacting that all the components cannot be machined closely enough to the nominal target dimension to be interchangeable in the assembly process. The select fit process, therefore, measures each component individually. Then it is determined which components can be used together to meet the dimensional tolerance requirements. It was recognized, however, that even with the use of the select fit process, a completely assembled injector exhibited timing, quantity and delivery variations that were higher than was acceptable to achieve performance and emissions goals.
The present invention is directed to overcoming one or more of the problems as set forth above.
According to one aspect of the present invention, a method is provided for assembling an apparatus of the type including a set of one or more input parameters, a set of one or more control parameters, and a set of one or more observed resultant parameters. The method comprises the steps of assembling a preselected number of components into the apparatus, performing tests on the apparatus subassembly to measure the values of the set of input parameters, determining one or more cumulative variation parameters using the set of input parameters for the assembled preselected number of components, determining the values of the set of control parameters using the set of cumulative variation parameters to compensate for the cumulative variation of the set of observed performance parameters exhibited for the preselected number of components, selecting, for each control parameter, a respectively associated component having an actual characteristic substantially equal to a respective desired characteristic wherein the respective desired characteristic is a function of the determined control parameter value, and assembling the selected component into the apparatus.
According to another aspect of the present invention, a method is provided for assembling a fuel injector of the type including a set of input parameters comprising nozzle steady flow, a set of control parameters comprising poppet lift and air gap, and a set of observed performance parameters comprising timing and delivery. The method comprises the steps of assembling a preselected number of components into the injector, performing tests on the injector subassembly to measure the values of the set of input parameters, including nozzle steady flow, determining, for both timing and delivery, a cumulative variation parameter using the set of input parameters for the assembled preselected number of components, determining the values of the set of control parameters, including poppet lift and air gap, to compensate for the cumulative timing and delivery variation exhibited for the preselected number of components, selecting, for each control parameter including poppet lift and air gap, a respectively associated component comprising a poppet lift shim and an armature having an actual dimension substantially equal to a respective desired dimension wherein the respective desired dimension is a function of the determined control parameter value, and assembling the selected component into the fuel injector.
According to another aspect of the present invention, a method is provided for assembling a fuel injector which includes a plurality of components, each component having an actual dimension, the injector being of the type including a preselected set of observed performance parameters comprising injection timing and delivery, a plurality of control parameters wherein changes in each control parameter value are effective to vary, by a predetermined amount, the value of each observed performance parameter, and wherein changes in the actual dimension of each component are effective to vary the respectively associated control parameter value. The method comprises identifying those control parameters for which the respective predetermined amounts are relatively large and for which the component associated with the identified control parameter is assembled relatively near the end of the assembly process, selecting, for each identified control parameter including poppet lift and air gap, components whose actual dimension is sufficient to reduce end of line timing and delay variation, and assembling the selected components into the fuel injector.
The present invention provides a method of assembling a fuel injector to reduce the injection timing and delivery variation of the final assembled injector, as required to meet emissions and performance goals by compensating for the fuel injection timing and delivery variation caused by, for example, the dimensional tolerance variations of certain components without affecting other performance parameters.